The present invention relates to an optical detector with shielding against electromagnetic interference.
Optical detectors are used, among other applications, in remote controls for converting optical signals emitted e.g. by an infrared emitting diode (IRED) into electrical signals. Because the remote controls should be able to function over long distances and thus react very sensitively to optical radiation, they must be screened against electromagnetic interference generated by, for example, switched mode power supplies. For this reason, previous assemblies have used a semiconductor photodiode with an n-type doped substrate which has a p-type doped area on the upper-side. The p-type doped area on the upper-side of the optoelectronic detector is connected to ground, and thus protected against electromagnetic interference. The remaining surface of the n-type doped substrate, in particular the side surfaces and the back are to some extent protected against electromagnetic influence by using additional mechanical shields. Such components have a solid or latticed metal shield which is arranged around the optical detector as an additional part in the inside. In this way, the detector is inset in a trough-shaped recess which is arranged in the mounting frames and closed by an optically transparent cover.
The disadvantages with this arrangement are that an additional case part is required, and the dimensions of this type of shielded optical detector are very large. Moreover, the angular characteristic of the assembled component is restricted by the trough and cover required. Another disadvantage is that a very large part of the semiconductor detector remains sensitive to electromagnetic interference.
In EP 0866 503 A2, an optical detector is disclosed in which an active, n-type doped, light-sensitive semiconductor area is arranged on a p-type doped semiconductor substrate, whereby a further highly p-type doped semiconductor layer is arranged on the surface of the active semiconductor area which covers the active semiconductor area in the shape of fingers. This additional semiconductor structure serves as a protective shield against the electromagnetic interference. Finally, the upper-side, including the protective shield, is covered by an insulating layer.
The disadvantage of this assembly is the technically difficult manufacture, in which additional doping has to be brought into the semiconductor materials. Furthermore, such an optical detector assembly has an additional large capacitance.
The object of the invention is to describe an optical detector which is protected against electromagnetic influence, without an additional external shield, and which can be manufactured simply and inexpensively without additional semiconductor processes.
The object of the invention is achieved in an optical detector according to the invention in that the optical detector with a p-type doped substrate has a coating of a conductive, transparent material which serves as a protective shield against the electromagnetic interference, and which carries away the charges generated by the electromagnetic interference. An insulating layer is arranged between the coating of conductive transparent material and an n-type doped active region provided on or embedded in the p-type substrate. The coating of conductive transparent material physically and electrically contacts a doped and preferably highly p+-type doped layer of the substrate to conduct away the charges.
The advantages of the invention are that such a type of electromagnetic protection is very inexpensive and can be attached by simple means during the manufacture of the wafer crystal. The surfaces which are sensitive to electromagnetic interference are reduced in size. This method does not need any additional finger-shaped semiconductor structures. Furthermore, it eliminates the need for separate parts for shielding the detector, so that detector devices, which are shielded against electromagnetic interference, in particular remote controls, can be inexpensively assembled in smaller packages. Moreover, troughs and covers, which unnecessarily reduce the angular characteristic of the detector unit, are no longer needed for the further assembly in a package. Furthermore, simpler mounting frames can be used. Finally, one bonding process is eliminated from the assembly of the detector.
Advantageous further developments include the following features. Conductive polysilicon or ITO (indium tin oxide) is used for the coating with which the upper-side of the detector is coated. The overhead capacitance of the assembly is reduced if an SiO2 insulating layer is additionally arranged between the active layer and the polysilicon coating, and if furthermore the polysilicon layer is electrically connected to the substrate via a highly p+-type doped semiconductor layer. The LTO (low temperature oxide) process is suitable for applying the oxide layer. It also proves to be advantageous if, on the back of the substrate, there is a highly p+-type doped semiconductor layer which ensures a good ground contact. In order to ensure a sufficient coating of polysilicon on the front surfaces of the oxide layer, the edges of the oxide layer are not rectangular but beveled. These beveled edges are created by taper etching.
The invention is explained in the following by means of an embodiment.